Concrete Footing Calculator

Wall, stem wall, pad, and pier footings: concrete, bags, rebar, and sizing

Project Details
Enter your measurements and requirements
Results
Calculated estimates for your project
Footing run40.0 ft
Concrete, with waste1.45 cu yd (39.1 cu ft, 1.11 m³)
Premix bags66 x 80 lb, 87 x 60 lb, or 131 x 40 lb
Bags or ready-mix?Above about 1 cu yd, ready-mix is usually cheaper and far less work.
Continuous bars2 #4 bars, 84 ft with 20 in laps
Rebar to buy5 sticks of 20 ft (84 ft), 20 chairs

Turn these numbers into a build plan.

  • 1.45 yd³ concrete
  • 66 80 lb bags
  • 5 sticks rebar
Materials List
Recommended materials and quantities
Concrete1.45 cu yd ready-mix
#4 rebar, 20 ft5 sticks
3 in chairs20
Form boards80 linear ft
grade16 in wide, 8 in thick

Full bar-by-bar detail is in the footing rebar calculator; for a shed or outbuilding load, start with the foundation load calculator.

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Project plan · built from your numbers

Pour 40 ft of wall footing

Dig, pass inspection, form and pour 40 ft of wall footing: 1.45 yd³ of concrete with waste and 5 sticks of #4 rebar.

Materials
$408
Time
1 weekend plus cure
Level
Intermediate
5 steps 6 tools 5 materials
  1. 1

    Lay out and dig

    String the footing lines, then dig to undisturbed soil and below the frost line.

    • Batter boards and strings set
    • Trench to depth
    • Bottom flat and clean
  2. 2

    Footing inspection

    Most jurisdictions inspect the trench, forms and steel before any concrete goes in. Book it early.

    • Permit posted
    • Inspection booked
    • Inspection passed
  3. 3

    Form and set steel

    Set forms level, put the bars on 3 in chairs, 2 continuous bars lapped 20 in at splices.

    • Forms level and braced
    • Steel on chairs with 3 in cover
    • Dowels set
01 · Your numbers
Concrete
1.45 yd³
80 lb bags
66
Rebar
5 sticks
Est. cost
$408
02 · The plan

5 steps · 6 tools · 5 materials

2 more steps, the full tool list and a shopping list you can check off as you go.

03 · Finished

Your plan is 60% built. Save it.

3/5

Pour 40 ft of wall footing

Finished in 1 weekend plus cure · $408 DIY

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How to calculate concrete for a footing

  1. Convert to feet. Divide the width and thickness in inches by 12. A 16 x 8 in footing is 1.333 x 0.667 ft.
  2. Multiply by the run. Width x thickness x length gives cubic feet: 1.333 x 0.667 x 40 ft = 35.6 cu ft. For a building, use the centerline perimeter, 2 x (L + W) minus 4 footing widths, so corners count once.
  3. Add waste. Add about 10% for uneven trench bottoms: 39.1 cu ft.
  4. Convert to yards or bags. Divide by 27 for cubic yards (1.45) or by 0.60 for 80 lb bags (66). Above about 1 cu yd, order ready-mix.
  5. Take off the rebar. Two #4 bars for footings up to 20 in wide, three up to 30 in, lapped 20 in at splices and held 3 in off the soil: 5 sticks of 20 ft for this run.
Worked exampleConcrete with 10% waste80 lb bagsRebar
40 ft continuous footing, 16 x 8 in1.45 cu yd665 sticks #4
24 x 24 ft garage, footing + 8 x 24 in stem wall8.21 cu yd37016 sticks #4, 24 dowels
Eight 24 x 24 x 12 in pads1.30 cu yd593 sticks #4

How big should a footing be?

Minimum width under IRC Table R403.1(1), in inches, by stories and soil bearing. The footing is at least 6 in thick, and its bottom sits at least 12 in below grade and below the frost line. The rule of thumb on top: width twice the wall, thickness equal to the wall.

WallSoil1 story2 stories3 stories
Wood frame1500 psf12 in15 in23 in
Wood frame2000 psf12 in12 in17 in
Wood frame3000 psf12 in12 in12 in
Block or brick1500 psf12 in21 in32 in
Block or brick2000 psf12 in16 in24 in
Block or brick3000 psf12 in12 in16 in
RegionTypical frost depth
Gulf Coast, Florida, southern California0 to 12 in
Mid-Atlantic, South, Pacific Northwest12 to 24 in
Ohio Valley, central Plains24 to 36 in
Upper Midwest, New England, Rockies42 to 60 in or more

Approximate. Your building department sets the frost depth and may require an engineered footing.

Rebar in footings

ItemTypical
Continuous bars2 #4 up to 20 in wide, 3 up to 30 in, 4 wider
Cover3 in from soil, on chairs
Laps40 bar diameters (20 in for #4, 25 in for #5)
CornersOne bent bar per continuous bar, each leg one lap long
Dowels into a stem wall#4 at 24 to 48 in, hooked in the footing
PadsA mat of bars at about 12 in each way

For bar-by-bar counts, use the footing rebar calculator. Round piers and columns get a cage from the concrete column calculator or a tube count from the sonotube calculator. Pouring a slab with a thickened edge instead? Use the concrete slab calculator. Then follow the concrete cheat sheet on pour day. Deck footings? Depth and size rules are in the deck building cheat sheet span tables.

Concrete footing calculator FAQ

How much concrete do I need for a 20 ft footing?

A 20 ft footing 16 in wide and 8 in thick is 17.8 cu ft, or 0.66 cu yd before waste. At 24 x 12 in it is 40 cu ft, about 1.5 cu yd.

How many bags of concrete for a footing?

Divide the cubic feet by 0.60 for 80 lb bags or 0.45 for 60 lb. A 40 ft run of 16 x 8 in footing takes about 66 bags of 80 lb with 10% waste, which is the point where ready-mix is cheaper.

How deep should a footing be for a block wall?

The bottom must sit at least 12 in below undisturbed grade and below your frost line, often 36 to 48 in in cold climates. The footing itself is usually as thick as the block (8 in) and twice as wide (16 in), wider for two stories.

What size footing for a retaining wall?

Retaining walls need a wider footing on the soil side, often 2/3 of the wall height for a cantilevered wall, plus drainage. Short garden walls can use a 16 to 24 in gravel or concrete footing; anything over 4 ft usually needs an engineer.

What size footing for a brick wall?

A single-story brick or brick veneer wall needs at least a 12 in wide footing on most soils under IRC Table R403.1, and 16 to 21 in for two stories on soft soil. A garden brick wall is commonly 2 to 3 times the wall width and 6 to 8 in thick.

Do footings need rebar?

The IRC does not require it for plain residential footings in low seismic areas, but most inspectors and engineers expect two continuous #4 bars, and it is required in seismic design categories D0 to D2 and under stem walls with dowels.

How much does a concrete footing cost per linear foot?

DIY materials for a 16 x 8 in footing run about $7 to $10 per linear foot (concrete, rebar and form boards). Contractors charge roughly $15 to $30 per linear foot including excavation.

Footing vs stem wall vs monolithic slab?

A footing spreads the load on soil. A stem wall sits on it and lifts the floor above grade, for crawlspaces or raised slabs. A monolithic slab pours a thickened edge footing and the floor together, common for garages; use the concrete slab calculator for that.

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